1. Field of the Invention
This invention relates generally to processes and equipment for removal of acid gases, such as CO.sub.2 and H.sub.2 S from gas mixtures. In particular, the invention relates to an improved method and apparatus for reclaiming heat from the rich alkaline scrubbing solution which has been used to absorb the acid gases contained in the gaseous mixture.
2. Overview
Absorption of an acid gas by an alkaline scrubbing agent causes the rich scrubbing solution temperature to increase by virtue of the heat of reaction between the acid gas and the alkaline agent. However, absorption of the acid gas by the scrubbing solution lowers its boiling point. As a consequence, when the rich scrubbing solution is introduced into the regenerator where it is subjected to steam stripping, a significant amount of water and acid gas in the rich scrubbing solution flashes and goes overhead from the lowered boiling point scrubbing solution. Flash of water vapor and acid gas from the rich solution results in significant loss of energy from the rich scrubbing solution that would otherwise be available to produce steam for steam stripping makeup, or for export.
The present process therefore, provides an improved method and apparatus for recovering heat of reaction energy from the rich scrubbing solution in a manner which precludes significant loss of energy from the alkaline scrubbing agent as a result of water vapor and acid gas flash from the rich scrubbing solution when it is introduced into the regenerator.
3. Description of the Prior Art
Processes and equipment are well known for removing CO.sub.2 and/or H.sub.2 S from gas streams which contain such acid gases. There are various industrial and commercial sources of gas mixtures that contain acid gases. Sour natural gas is one source. The gas mixture resulting from enhanced oil recovery processes which utilize CO.sub.2 also includes a quantity of acid gases. In addition, the low temperature shift effluent from a steam reformer reaction associated with an ammonia plant, contains substantial quantities of carbon dioxide and/or hydrogen sulfide. The process off gas in this instance leaves the processing equipment at a relatively high temperature and pressure, and usually contains significant amounts of steam as well. Many partial oxidation reactions also produce a gaseous effluent containing acid gases.
The acid gases are removed from the gas stream by absorption of the CO.sub.2 and/or H.sub.2 S in a hot aqueous alkaline scrubbing medium such as potassium carbonate, which is then regenerated by steam stripping. After passage of the gaseous mixture through the scrubber containing an alkaline scrubbing agent, the purified gas goes overhead while the rich scrubbing solution underflow is returned to a regenerator for steam stripping thereof.
The process of removing CO.sub.2 and/or H.sub.2 S from gas streams containing these acid gases can be made more efficient if thermal energy is recovered from the absorption medium in order that the reclaimed heat can be used to produce steam for use in regeneration of the hot alkaline scrubbing agent. Reduction of the regeneration heat requirements has been a much sought after goal. Production of significant quantities of steam over and above that required for steam stripping of the rich scrubbing solution to provide export steam represents a further advantageous result.
Eickmeyer in U.S. Pat. Nos. 3,932,582, 3,851,041 and 4,271,132 issued in 1976, 1974 and 1981 respectively taught that process waste heat contained in the raw gas stream may be recovered by condensing most of the steam before the gas stream is introduced into an absorber containing an alkaline absorption agent. Eickmeyer, however, did not suggest recovery of heat from the rich scrubbing solution leaving the absorber.
U.S. Pat. Nos. 3,823,222 to Benson, and No. 4,160,810, issued to Benson and McCray also teach that steam may be recovered from the hot process gas prior to passage of such gas through an alkaline agent absorber for use in stripping the rich scrubbing solution leaving the absorber. In these patents, the hot rich scrubbing solution is sent directly from the absorber to the regenerator with no suggestion being made that waste heat can efficiently be removed from that solution.
Osman in U.S. Pat. No. 4,409,191 teaches a cyclic process for the removal of acid gases from a feed gas stream comprising a hot, steam-containing gas mixture by means of an aqueous alkaline scrubbing solution which is continuously recycled between an absorber in which acid gases are absorbed by scrubbing solution and a regenerator wherein acid gases are desorbed by steam stripping. Stripping steam is at least partially derived from the hot, steam-containing gas mixture. Condensation and separation of the steam content of the hot process gas is carried out before the gas mixture enters the absorber. However, the Osman process does not teach or suggest reclaiming of heat from the rich scrubbing solution that is derived from the heat of reaction of the acid gas with the scrubbing agent and makes no mention of using such steam as a part of the steam makeup for stripping of the rich scrubbing solution introduced into the regenerator.